KR920009844B1 - Plated steel sheet having excellent coating performance - Google Patents

Abstract

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Description

[Name of invention]

Plated steel sheet with excellent paintability

Detailed description of the invention

[Technical Field]

The present invention relates to a plated steel sheet having excellent coating appearance when used in cation electrodeposition coating applications such as in the case of components of automobile bodies.

[Background]

In North America, Canada, Northern Europe, etc. in order to prevent road freezing in the winter, rock salt is applied to the steel plate used as a member of the vehicle body excellent corrosion resistance performance is required.

For this reason, a pure zn plated steel sheet or Zn-based alloy plated steel sheet (for example, Zn-Fe-based alloy plated steel sheet, Zn-Ni-based alloy plated steel sheet, etc.) having excellent corrosion resistance has recently been used. However, these plated steel sheet has a problem that the coating appearance is inferior due to the cratering of the coating film when the cathodic electrodeposition coating is performed after the phosphate treatment in the single layer plating. Therefore, as a plated steel sheet which improves such a coating appearance problem, a multilayer plated steel sheet which is subjected to Fe-based plating having good electrodeposition coating property on pure Zn or Zn-based alloy plating is proposed.

Known conventionally for this type of steel sheet is that the upper layer is composed of Fe-Zn-based alloy plating having a Fe content of 60-90wt%, and is composed of Fe plating, and these multilayer coated steel sheets are subjected to cationic electrodeposition coating. Certainly, the occurrence of cratering of the coating film is reduced and the coating appearance is improved. However, in order to reduce the occurrence of cratering in the coating film by Fe-Zn-based alloy plating having a Fe content of 60-90 wt%, the plating cost must be 5 g / m2 (one side) or more, so the manufacturing cost is high. In addition, since the Fe-based alloy plating is hard and brittle, numerous cracks are formed when the plated steel sheet is processed into a member. As a result, the underlying layer is exposed at the crack. Therefore, in the case of carrying out electrodeposition coating, electrodeposition coating was directly processed on the lower layer, and cratering was easily generated in the coating film.

On the other hand, since Fe plating is more flexible than Fe-Zn alloy plating, cracking does not occur even when the plated steel sheet is processed into a member, and there is little cratering in the coating film.

However, when the plating deposition amount is less than 3 g / m 2 (one side), variation occurs in the amount of cratering. Although the cause of this change is not clear, it is easy to generate large phosphate crystals when phosphate treatment is performed before electrodeposition coating because the coating rate of upper plating to lower plating is poor or the purity of upper plating is high. As a result, the coverage of the surface of the plating layer due to phosphate crystals decreases, and the coverage also fluctuates so that the electrolytic conduction at the time of electrodeposition coating is changed between the portion where the phosphate crystal is deposited and the portion which is not precipitated. Judging.

Therefore, in order to reduce the occurrence of cratering in the coating film by Fe plating, the coating weight should be 3 g / m 2 (one side), and as a result, the cost becomes higher.

As described above, the present invention is not perfect in electrodeposition coating property even in a multilayer coating steel sheet obtained by performing Fe-based plating on pure Zn coating or Zn-based alloy plating. The purpose is to provide a plated steel sheet which is improved and inexpensive to manufacture.

[Initiation of invention]

The present invention provides a plated steel sheet, which is composed of a lower layer coating of pure Zn or Zn based alloy formed on a steel sheet, and a lower layer coating of the plated steel sheet. Formation of upper layer coating of Fe-Zn-based alloy having B (boron) -containing Fe or B-containing Fe% (boron-containing Fe percentage) of 50wt% or more, and the occurrence of cratering on the coating film Consists of forming a plated steel sheet to reduce the amount of upper layer plating.

In the plated steel sheet of the present invention, the electrodeposition coating property is excellent in that when B is added to the bath during the upper layer plating, the uniform electrodeposition of the plating is improved to make the plating layer uniform, and the upper layer contains B. If phosphate is treated as an electrodeposition coating, they become nuclei when phosphates are deposited to form a fine and dense phosphate coating, resulting in uniform electrical conduction of the entire surface of the steel sheet. It is judged that the coating material is uniformly electrodeposited when it is made, and it can stably suppress the occurrence of cratering.

[Best form for carrying out the invention]

In the coated steel sheet of the present invention, when the upper layer is B-plated Fe plating, the plating deposition amount is 3g / m ^÷屋 per side, and in the case of the element-containing Fe-based alloy, the plating deposition amount is 5g / m2 or less per side. The occurrence of cratering is suppressed.

This is considered to be able to improve the lower coating rate by the upper layer because of the excellent coating property to the lower layer of the upper layer even if the amount of plating deposition.

In the case where the upper layer is composed of Fe-Zn alloy plating, conventionally, if the Fe content is not adjusted to less than 60-100 wt%, the occurrence of cratering in the coating film cannot be reduced. Reducing to less than 50-100 wt% can also reduce the occurrence of cratering in the coating film. The reason for this is presumably due to the effect of improving the phosphate treatment by B. However, once the Fe content is reduced in this way, the corrosion potential difference between the upper and lower layers is reduced, and the long-term corrosion resistance of the entire plating layer can be improved. It is preferable to make B content rate contained in the upper layer into 0.001-3 wt%. This means that when the B content is less than 0.001 wt%, the occurrence of cratering in the electrodeposition coating and its fluctuations are not different from those of the upper layer containing no B. The effect is saturated even when the B content exceeds 3 wt%. This is because increasing the amount of B content is meaningless.

The plating deposition amount (one side) of the upper layer is preferably adjusted to 0.5 g 10 g / m 2 for B-containing Fe plating and to 0.2-8 g / m 2 for Fe-based alloy plating.

This is because if the plating amount is less than 0.5g / m2 (for B-containing Fe plating) or 0.2g / m2 (for Fe alloy plating), the lower layer cannot be exchanged and the lower layer is exposed during electrodeposition coating. When cratering occurs in the coated film and the plating amount exceeds 10 g / m2 (for B-containing Fe plating) or 8 g / m2 (for Fe alloy plating), the effect of suppressing the occurrence of cratering in the coated film is saturated. This is because the adhesion amount does not need to be larger than this.

In the case of the upper layer plating composed of the conventional Fe-Zn alloy containing no B, the saturation of the effect of suppressing the occurrence of cratering in the coating film is 10 g / m 2, but the plating amount by the addition of B is also advantageous. Reduction of adhesion amount is recognized. In order to contain 0.003-3wt% of B in the upper layer by the electroplating method, boric acid, metaboric acid, water-soluble meta in a conventional Fe plating bath or Fe alloy plating precursor such as Fe-Zn, Fe-Zn, Fe-Ni, etc. One or two or more boron compounds such as borate, water-soluble tetraborate, and tetrafluoroborate may be added and plated by adjusting the pH of the bath to 1-3.

The plated steel sheet according to the present invention has improved electrodeposition coating properties of pure Zn-plated or Zn-based alloy plated steel sheet, but when the lower layer is made of Zn-based alloy of Zn-Ni-based or Zn-Fe-based alloy, For the Zn-Fe alloy, even if it contains a small amount of one or two or more elements such as Fe (but Zn-Ni alloy), Co, Cr, Mn, Mo, and Ti, the occurrence of cratering can be suppressed. When the lower layer is composed of pure Zn plating, even if the plating is performed by electroplating, vapor deposition, or hot dipping, the upper layer is suppressed. In the case of the alloy plating, in addition to plating in the above-described method, after hot dip galvanizing or evaporating zinc plating, such as alloyed zinc-plated steel sheet, the Zn-Fe-based alloy is thermally diffused. Suppress alloying Can be.

The following describes the present invention in detail by way of examples.

Example 1

The surface is cleaned by ordinary degreasing and pickling on cold rolled steel sheets with a thickness of 0.8 mm, and then, first, pure Zn or Zn-Fe alloys or Zn-Ni alloys under the conditions shown in the following Table 1 by electroplating. The lower layer plating was carried out, and then the upper layer plating of Fe and Fe-B was carried out by the same plating method thereon.

Subsequently, 10 samples were taken from the plated steel sheets plated in this manner and treated with commercially available phosphate treatment solution (Bt3030, Nippon Parker), followed by cationic electrodeposition coating and drying. After that, the number of craters generated in the coating film was examined. In the case of electrodeposition coating, using Electron # 9000 (manufactured by Kansai Paint Co., Ltd.) as a coating material, the coating film thickness was transferred to a thickness of 25 μm at 300v by the instantaneous voltage-increase method. Drying was performed for 20 minutes at 180 ° C.

In the second table shown below, the number of cratering occurrences of the coating film when Fe-B plating is applied to the upper layer is shown.

As apparent from Table 2, the plated steel sheet of the present invention subjected to Fe-B plating on the rise is more crater than the good electrodeposited coated plated steel plate coated with Fe on the conventional upper layer even if the lower layer is pure Zn plating or Zn alloy plating. The number of occurrences of the ring and its fluctuations are small.

[Table 1]

[Table 2]

Example 2

Cold-rolled steel sheets with a thickness of 0.8 mm were cleaned in the same manner as in Example 1, and then under-plated with Zn-Fe alloy or Zn-Ni alloy under the conditions shown in Table 3, and then on B An upper layer of a high Fe—Zn alloy containing or a high Fe—Zn alloy containing B was performed.

Next, a sample was taken from this plated steel sheet and subjected to phosphate treatment and electrodeposition coating in the same manner as in Example 1 to obtain a coated steel sheet having a coating thickness of 23 μm.

Table 4 shows the number of craterings generated in the coating film when B-containing high Fe-Zn alloy coating was applied to the upper layer.

[Table 3]

[Table 4]

Example 3

Steel strips with a plate thickness of 0.6 mm and a width of 300 mm are reduced in a gas reduction pretreatment oven, followed by a negative pressure chamber and a number of seal rolls to prevent ingress of gas or air. Pass through a seal rollchamber, which is gradually depressurized, to a first deposition plating chamber in which a vacuum deposition Zn bath of a battery resistance heating method is disposed below the band steel position. Pure Zn was deposited by plating.

Subsequently, the strip steel is guided to a second deposition plating chamber having the same structure as the first deposition plating chamber disposed below the first deposition plating chamber, and deposited pure Zn on the opposite side thereof, and then passes through the seal roll chamber and the negative pressure chamber. To prepare a deposited steel strip having a plating weight of 50 g / m 2 (one side). The plating was performed under a condition of a steel strip moving speed of 15 m / min and a vacuum degree of 0.01 torr of the first and second deposition plating chambers.

Next, a portion of the pure Zn-plated steel strip thus prepared was put in an oven of a mixed gas (dew point: -25 ° C) of H ₂ 3% and N ₂ 97%, and heated to 280 ° C. to coat the plated layer with Zn-Fe alloy. It was produced as a strip steel composed of Zn-Fe alloy plating with Fe% of 10 wt%.

Thereafter, the Zn-Fe alloy strip steel and the pure Zn pure steel strip steel were subjected to Fe plating or Fe-B plating under the same conditions as the upper plating shown in Table 1, or B-containing high Fe under the same conditions as the upper plating in Table 3. -B alloy plating was carried out by the electroplating method and electrodeposition coating was carried out under the same conditions as in Example 1.

Tables 5 and 6 show the state of cratering in the coating film.

[Table 5]

[Table 6]

Industrial availability

The coated steel sheet of the present invention can be used for other members such as household electrical appliance members, building members, and the like, which are subjected to electrodeposition coating, in addition to automobile body members, thereby obtaining a favorable coating appearance. Moreover, it is excellent in the coating property of coating materials other than electrodeposition coating material, and can be used also as a general coating material.

Claims (5)

Underlayer plating of pure Zn or Zn-based alloy is applied to the steel plate, and Fe plating containing Fe (B) or Fe-Zn containing B (boron) is applied on the lower layer plating. Plated steel sheet with excellent paintability. The plated steel sheet having excellent coatability according to claim 1, wherein the upper plating is Fe plating containing 0.001-3 wt% of B. The plated steel sheet having excellent paintability according to claim 1, wherein the upper plating is Fe-Zn alloy plating containing 0.001-3 wt% of B. The coated steel sheet according to claim 1 or 2, wherein when the upper layer is Fe-plated containing B, the plating adhesion amount is 0.5-10 g / m 2 per one side. The plated steel sheet with excellent coating property according to claim 1 or 3, wherein when the upper layer is Fe-Zn alloy coating containing 50 wt% or more of Fe containing B, the coating weight is 0.2-8 g / m 2 per one side.